If you add a custom.less file in your customisation to style it, does it overwrite the CSS of all other customisations?*

Sugar documentation says custom CSS/LESS styling should be put into custom/themes/custom.less, but it does not state anywhere that this code will be merged with the existing custom/themes/custom.less code.

How is that supposed to work and what is the real intended way to add CSS in customisations? Straight in the handlebars templates?